Of what I searched in, I haven't found a clear indication as to the answer of this question: When a creature is Hidden from another creature, let's call them "Heidie" and "Attacky" respectively, does Heidie trigger any of Attacky's powers? In my last game session, two situations arose simultaneously:

Heidie was invisible and adjacent to Attacky. On Attacky's turn, she chose to ready an action to attack Heidie when she moved from the square. On Heidie's turn, she spent the first move action to become hidden from Attacky. She then shifted from the square.

Situation #1: Does Heidie trigger the readied action?

Situation #2: Attacky also had a power, Blurred Step, that triggers when an opponent shifts out from an adjacent square she may then shift 1 square. Does Heidie trigger that power?

2 Answers
2

Neither #1 nor #2 are triggered as long as the creature begins it's action hidden.

The reason for this mostly has to do with the fact that you are unsure of the position of the creature that is hidden. A teammate from whom the creature is not hidden from could relay their location to you, but that only goes so far, for one thing, it does not change the fact that the creature is invisible/undetectable to you, you still only know it's location when you are told. If the creature is hidden from you, you can neither see it, nor detect it. Namely, you don't know when it moves out of it's square, thus, you don't know when the readied action triggers.

As far as situation #2, in this case the monster shifting away would not trigger the power. Again, you don't actually know the location of the creature that is now invisible and hidden. And you don't know when it moves. The argument that a teammate could inform you of the creatures movement is actually an interesting one. But free actions resolve as reactions so you don't know the creature has left until it's too late to act.

WRT to your second statement, do recall that in my example Heidie started her turn invisible. So she only needed a regular move action to become hidden and the other move action to shift. None the less, I'm leaning towards your interpretation.
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Trey KirkJan 3 '13 at 17:57

Situation 1: When hidden, creatures from which you are hidden must guess what square you are in. This means that you do not know which square a creature is in, nor when the creature moves. In this example Attacky doesn't know that Heidie is still adjacent to her as soon as she becomes hidden. When Heidie moves away, Attacky still doesn't know which square Heidie is in, nor that Heidie has moved. Therefore, the triggering condition "attack when Heidie moves from the square" cannot be triggered until Attacky senses (sees, hears, etc.) Heidie moving.

Situation 2: This one is a little bit more complicated, depending on what the power actually is. The definition of Invisible states:

If a creature is invisible, it has several advantages against creatures that can’t see it: It has total concealment against them, it doesn’t provoke opportunity attacks from them, and they grant combat advantage to it.

The key here is that invisible creatures don't provoke opportunity attacks, however nothing is stated about immediate or free actions with triggers. In this example, it sounds like Blurred Step:

Trigger: An adjacent enemy marked by you shiftsEffect: You shift 1 square.

In this case, the trigger doesn't specify that you need to see the marked enemy, only that it shifts. A strict interpretation is that, even if you don't know which square the marked enemy is in, so long as it shifts while being adjacent, you can shift 1 square. Of course, Attacky has no idea where Heidi shifted to, so would have to guess which way to shift.

Amusing follow-on to this, if Attacky had Harrying Step:

Benefit: When you use your blurred step, you can teleport to any square adjacent to the triggering enemy instead of shifting.

Attacky would have to pick any square on the map and try to teleport to that square, and if it weren't adjacent to the triggering enemy (Attacky has no idea which square Heidi is in after the shift) the teleport would simply fail.

The Problem: The problem here is that Attacky still doesn't know that Heidi has shifted and Blurred Step is a power you have to actively use, rather than a feature that automatically activates. This is where the problem becomes one of meta-game: if both Heidi and Attacky are players, Attacky knows Heidi shifts from when the player says "I shift" and can react. If Heidi is controlled by the DM then the DM does not have to announce his or her intentions to shift Heidi and Attacky doesn't know when to use Blurred Step. I suppose, in theory, Attacky could keep shouting "I use Blurred Step!" until it activates.

On the other hand, if Attacky had a feat that said "any time a creature adjacent to you shifts, you shift 1 square" then Attacky would suddenly and unexpectedly feel the urge to shift one square and not really know why.